Rickettsia tsutsugamushi, the causative agent of scrub typhus, is noted for its diversity with regard to both serotype and virulence. These serotypic diversity confounds the establishment of protective immunity to scrub typhus and complicates its serodiagnosis. The molecular basis for R. tsutsugamushi strain variation will be examined by characterization of the structure and antigenicity of the outer membrane proteins. The goals of this study will be accomplished by (1) identifying conserved and variable regions of the major protein immunogen by immunoprecipitation and immunoblotting of proteins from different strains with monoclonal antibodies specific for identified epitopes and by comparison of protein fragments from different strains by acrylamide gel electrophoresis and immunoblotting; (2) correlation of epitopes with specific amino acid sequences of the quantitatively major, immunodominant (60K) protein by selection with monoclonal antibodies of bacterial colonies carrying gene fragments coding for certain epitopes; and (3) investigating the chemical nature of epitopes identified with monoclonal antibodies by testing their reactivity with antibody before and after protein denaturation or treatment to destroy protein or carbohydrate moieties. Thus, these studies will continue to probe the significance of outer membrane proteins of R. Tsutsugamushi in the antigenic variation of this species of intracellular pathogens. The proposed studies will help define scrub typhus components which should be useful for vaccine testing and for the preparation of immunodiagnostic reagents, and they will shed some light on the molecular basis for genotypic and phenotypic variation of these organisms.